This invention relates in general to polyimide foam structures and, more particularly, to methods of reshaping polyimide foam sheets into self-supporting structures of desired shapes.
Polyimide and modified polyimide foams are manufactured by a number of different processes. Typical are those described in our prior U.S. Pat. Nos. 4,407,980 and our co-pending U.S. patent applications Ser. No. 678,992 filed Dec. 6, 1984 and Ser. No. 556,604, filed Feb. 14, 1985. Also typical are U.S. Pat. No. 4,315,076 filed Apr. 16, 1981, U.S. Pat. No. 4,361,453 filed May 11, 1981 assigned to International Harvester and Monsanto respectively.
In one method, polyimide precursors are prepared by reacting a tetracarboxylic acid dianhydride or its half ester with a diamine, then heating the precursor in a thermal over to cause the material to expand into a foam while forming the polyimide. A large block or bun of foam is produced.
Another method of preparing polyimide foams involves reacting an imidocaproic acid monomer with a diamine. The imidocaproic acid monomer is prepared by reacting a tetracarboxylic acid dianhydride with caprolactam in a ratio of dianhydride to caprolactam of less than about 1:1.5. The resulting polyimide powder is caused to foam by heating it to a temperature in the range of about 150.degree. to 320.degree. C. in a square mold. The resulting foam bun is then cut into slabs or sheets of desired thickness.
In some cases it is possible to directly foam the polyimide to the desired sheet thickness. However, in most cases the resulting sheet has an excessively irregular upper surface, so that it is usually preferred to slice sheets from a larger block to obtain highly uniform sheets of the selected thickness.
Polyimide foam sheets have a large number of applications which take advantage of their flexibility and flame resistance, such as seat cushions in aircraft. Other applications such as insulating air ducts in aircraft, spacecraft, naval ships, etc., which need the flame resistance and lack of toxic gases when exposed to flame for the protection of passengers and personnel require a variety of foam configurations. Wrapping ducts with sheets is often not satisfactory, since the elastic characteristics of the sheets cause them to come loose and they are difficult to bend around sharp corners. Such sheets also are not adaptable to requirements for varying thicknesses, often are moisture permeable and have low surface strength.
Attempts have been made to foam the polyimide in a mold, such as a half-pipe, having a desired final configuration. This has been, unfortunately, found to produce structures of greatly varying density and poor mold filling, especially with molds having relatively thin areas. The structures produced by this method have been found to have local weak areas and to not retain their final shapes well. It is very difficult to obtain a desired density and strength uniformity (or selected density and strength gradients) in structures made by this method.
Thus, there is continuing need for improved methods of re-shaping polyimide foam sheets into desired shapes. Improvements are needed in density and strength uniformity. The shapes should be easily made, using standard foam sheets as the starting material. The ability to produce structures which are strong and retain their molded shape out of the mold is needed. A method which permitted forming foam structures having areas of different thicknesses and selected densities would be particularly useful.